Method for increasing the heating rate in continuous annealing processes

ABSTRACT

Alkali metal silicate coatings are applied to steel sheet or strip material to increase the emissivity of the material undergoing heating and/or cooling under substantially radiative conditions. The heating rate of the sheet material is markedly increased with the application of coating thicknesses within the range of about 5 to 20 milliounces/ft2.

O United States Patent 1 1 11 1 3,718,510 Patula et al. 1 1 Feb. 27, 1973 [54] METHOD FOR INCREASING THE 3,582,409 6/1971 Kohler ..l48/1 13 X HEATING E [N N INU 3,301,702 1/1967 Amcs et al.... ....148/14 X 3,480,486 11/1969 Tanaka et al..... ..l48/l4 X ANNEALING PROCESSES 2,906,649 9/1959 Keuth et al. ..l 17/ 135.] X [75] Inventors: Edward J. Patula, Monroeville; 2,628,175 2/1953 Henderson ..1 17/ 135.1 X Robert R. Somers, Penn Hills 2,462,763 2/1949 Nightingall ..1 17/ 135.1 X Township; William L. Roberts, 1,856,751 3/1932 Prefahl ..148/14 X Franklin Township, Westmoreland 1,750,305 3/1930 Gross ..1 17/ 135.1 X Cty., all of Pa. Primary Examiner-Charles N. Lovell [73] Assignee. Umted States Steel Corporation Atmmey Arthur J. Greif [22] Filed: Feb. 16, 1971 21 Appl.'No.: 115,456 [571 ABSTRACT Alkali metal silicate coatings are applied to steel sheet 521 US. Cl. ..14s/14, 117/1351, 148/18 strip material increase the emissivity the 148/28 148/29 material undergoing heating and/r cooling under sub- 51 Int. Cl. ..C2ld 1/48 Stamially radiative eenditiens- The heating rate of the 58 Field of Search ..14s 12.1, 14, 18, 122, 113, sheet material is markedly increased with the pp 148/28, 29; 117/135 1 tion of coating thicknesses within the range of about to 20 milliounceslft 56 R t d l 1 e 4 Claims, 1 Drawing Figure UNITED STATES PATENTS 3,208,874 9/1965 Conner, Jr ..1 17/1351 STRIP SPEED -/0 FPM HEflT/IVG RATE RATIO OF COATED T0 UNCOATED STRIP 60A TIA/G wE/aHr, MIL L IOUNCES/F r 2 STRIP THICKNESS 0.0066" HEAT/N6 ZONE TEMPERATURE-IJQOVI TO 1500",.

PATENTED FEBZYIQYS nttkm 0 3 E 0 w w w 0 T F a 0 w -2 3 4 E H 6 0 A 0 1w mm P mow ww 0 MDK E0 2 m P MS 0 WWW E 5 arr H88 0 COAT/N6 WEIGHT; MILL/OUA/CES/FT INVENTORS.

I EDWARD J. PATULA, WILL/AM L. ROBERTS 5 ROBERT R. OMERS METHOD FOR INCREASING THE HEATING RATE IN CONTINUOUS ANNEALING PROCESSES This invention relates to a method of continuous annealing, whereby both the heat-up rate and cooling rate of the steel sheet material undergoing recrystallization is markedly increased.

Continuous annealing is employed extensively in the production of recrystallized sheet and strip material. In this treatment, the strand of light-gage cold reduced metal, travels at high speed through a radiant heating zone, under a non-oxidizing atmosphere, where it is brought to desired temperature in a short time, held at temperature for a period of a few seconds, passed through a cooling zone and then emerges into the air at a temperature below that at which it rapidly oxidizes.

Since both the heating and cooling of the strip are effected primarily by radiation, the heating and cooling rates are dependent on the emissivity of the surface of the strip. An ideal black body has an emissivity of 1.0, whereas the ordinary steel strip employed in such continuous annealing processes has an emissivity below 0.2, resulting in much slower heat transfer rates than could be obtained with surfaces having higher emissivities. Consequently, a considerable length of heating and cooling section is required to achieve the desired temperature; and for commercial lines operating at speeds of about 1,500 ft/min., these sections are necessarily large and expensive. If the emissivity of the strip could be effectively increased prior to its entry into the heating zone of the furnace, continuous annealing lines operating at a fixed speed could be reduced in size and cost. Conversely, lines could be made to operate at even higher speeds and on thicker strip. It has now been found that certain alkali metal silicates, when applied to the steel surface, in certain critical coating thicknesses, not only effectively increase the emissivity of the surface, but also possess a combination of other properties desirable for the commercial utilization of such a technique.

It is therefore an object of this invention to provide a method for coating strip material to markedly increase the emissivity of the surface thereof.

It is another object of this invention to provide such a coating method with a minimum of additional cost.

It is still another object of this invention to provide a coating method which does not affect appearance or end use of the strip.

These and other objects will be more apparent when read in conjunction with the following description, taken in conjunction with the appended claims, and the FIGURE which shows the critical effect of sodium orthosilicate coating thickness on strip heating rate.

The di, meta, and ortho silicates of sodium and potassium have been found to possess a combination of properties (hereinafter described) which make them particularly suitable as coating materials for increasing e. they are able to withstand roll contact without chipping or flaking and are therefore amenable to thickness control; and

f. they provide significant increases in heat-up and cooling rates.

The coatings may be applied as a wet cake, by brushing, by spraying of an aqueous mixture or by passage through a bath. In the latter, preferred method, the cold reduced strip is conveyed through a tank containing an aqueous mixture (slurry, suspension, or solution) of the silicate. It is then passed through rolls to control the uniformity and thickness of the coating, followed by drying jets to remove the greater part of the water. In an alternate procedure, the rolls may be eliminated, and the drying jets may be employed to serve both functions, by serving as an air knife (analogous to the procedure of, for example, U.S. Pat. No. 3,459,587). Subsequent to drying, the strip with the proper thickness of coating on the surface thereof, is passed to the annealing furnace, to effect recrystallization of the cold reduced material. Following recrystallization, the strip passes to a cooling section, where it is permitted to cool under a controlled atmosphere with the high emissivity coating still intact. As the strip emerges from the cooling section at a temperature cool enough to avoid oxidation, it is cooled by water jets, which serve a dual function by dissolving and removing the silicate coating.

In the investigation which led to this invention, potential coatings were evaluated, using a muffle furnace with a specially designed protective atmosphere chamber. Samples were evaluated for their ease in the application and removal of the coating as well as for their emissivity. In this regard, it should be noted that materials need not be dark in appearance to have high values of emissivity. For example, asbestos, glass, and white enamel paint, although light in appearance had significantly increased emissivities. Coatings were applied by brushing and then drying completely.

Samples (4 X 8 X 0.0085 inches) were screened by determining the heating rates from F to a temperature of l,l00 F. In some tests, thermocouples were attached to both the coated and uncoated portions of the same sample; both temperatures being simultaneously and continuously recorded-In other tests, separate samples of coated and uncoated strip were prepared and tested. Of the coatings tested, which included: MgO slurry; CaSO 2I-I O; H PO,; BOP furnace filter cake; phosphate cleanser; potassium chromate, iodide and fluoride; sodium chloride, and fluoride; iron chloride; chromium chloride; and sodium silicate (water glass) only the di, meta, and ortho silicates of sodium and potassium possess the ability to significantly increase the emissivity of the strip without marring its finish, and to be easily and economically removable by simple rinsing.

Although even a very thin coating of silicate would effect at least some increase in emissivity, it has been found that there exists a critical minimum thickness for achieving a significant increase in heat-up rate. Referring to the FIGURE, it may be seen that as coating weight increases, the heating rate ratio (the heating rate of coated strip, divided by the heating rate of uncoated strip) also increases. However, as evidenced by the FIGURE, this ratio increases rapidly above a thickness of about 5 milliounceslft reaching a maximum at about milliounces/ft of strip surface, and thereafter decreases, but more slowly. Thus, while very thick coatings would still provide significant increases in heat-up ratio, coatings greater than about 20 milliounces/ft would be less desirable both because of increased material cost as well as increased cost of removal. Coatings within a range of about 9-15 milliounces/ft being preferred, for providing both maximum heat-up rates as well as minimum cost.

Pilot line tests have been run to compare the service performance of control strip with that of strip annealed with the sodium orthosilicate coating of this invention. These comparison strips were both chrome plated and tin plated and then processed through standard quality performance tests. For the chromium plated product, these tests included the stack rust test, humid storage test, salt-citric acid test and accelerated salt-water test, whereas for tin plated strip, the tests included the stack rust test, iron solution value, alloy tin couple and pickle lag testing. In all cases, the precoated annealed strip of this invention was equal to or better in quality performance than that of the control, untreated annealed strip. It is clear, therefore, that these emissivity coatings do not change the surface chemistry of annealed strip in any manner which would deleteriously affect the end use of the annealed product.

The term strand as used herein and in the appended claims is to be understood as including flat rolled metal articles of various dimensions, as well as wire, rod, or other elongated articles amenable to continuous radiation heat treatment.

We claim:

1. In the continuous recrystallization annealing of steel strand, wherein radiative heating is employed to supply at least a major portion of the required rapid increase in temperature, the improvement which comprises: applying to the surface of the strand, a coating selected from the group consisting essentially of the di, meta and ortho silicates of sodium or potassium, and mixtures thereof, in a thickness of from about 5 to about 20 milliounces per square foot of strand surface; thereafter, rapidly heating said strand to a temperature above the lower critical to effect recrystallization, passing said strand to a cooling zone and subsequently removing said coating from the strand surface.

2. The method of claim 1, wherein the coating is applied as an aqueous mixture of said silicate, and substantially all the water is eliminated from said coating prior to said rapid heating.

3. The method of claim 2, wherein said mixture consists essentially of an aqueous solution of said silicate.

4. The method of claim 3, in which the major ingredient in said solution is sodium orthosilicate, and said thicknesses from about 9 to about l5 milliounces per square foot. 

2. The method of claim 1, wherein the coating is applied as an aqueous mixture of said silicate, and substantially all the water is eliminated from said coating prior to said rapid heating.
 3. The method of claim 2, wherein said mixture consists essentially of an aqueous solution of said silicate.
 4. The method of claim 3, in which the major ingredient in said solution is sodium orthosilicate, and said thicknesses from about 9 to about 15 milliounces per square foot. 